Why Does Current Reflect in an Antenna System?

[david845]

antenna circuit theory help!

Im having trouble understanding vswr and current path in an antenna system. Example is the dipole...Ive read that the parasitic capacitance between the 2 wires forms the complete circuit but then I read about standing waves that reflect at the open circuit. My questions are...if capacitance completes the circuit why does current reflect back ti the source? How does electric current reflect and travel a different direction without emf? If at higher frequencies an open circuit is not actually open then why does the current get reflected back? If current can't flow in an open circuit how the heck did it start flowing toward the load in the first place?

 

[berkeman]

Im having trouble understanding vswr and current path in an antenna system. Example is the dipole...Ive read that the parasitic capacitance between the 2 wires forms the complete circuit but then I read about standing waves that reflect at the open circuit. My questions are...if capacitance completes the circuit why does current reflect back ti the source? How does electric current reflect and travel a different direction without emf? If at higher frequencies an open circuit is not actually open then why does the current get reflected back? If current can't flow in an open circuit how the heck did it start flowing toward the load in the first place?

Welcome to the PF.

You only get a reflection back at the feedline/antenna terminal interface if there is a mismatch in the impedance between the Zo of the feedline cable and the input impedance of the antenna at the terminals. If they are matched, there is no reflection, and SWR = 1.

Do you know the feedpoint impedance of a dipole antenna at its fundamental resonance?

 

[david845]

I do not know that at all. I am confused if its not an open circuit at high frequency then why does an impedence mismatch which is essentially like a partially open circuit reflect the current? It seems like the description is double edged sword "its not an open circuit at high frequency so current flows but at the open circuit junction the current gets reflected. I guess it is like a capacitor but its the fields were concerned about and not actual currents?

 

[berkeman]

I do not know that at all. I am confused if its not an open circuit at high frequency then why does an impedence mismatch which is essentially like a partially open circuit reflect the current? It seems like the description is double edged sword "its not an open circuit at high frequency so current flows but at the open circuit junction the current gets reflected. I guess it is like a capacitor but its the fields were concerned about and not actual currents?

You can sort of think of the dipole as two inductors and a capacitor. Picture each element as an inductor, with a flat plate at the top that serves as half of the capacitor. The traveling RF wave coming down the coax feedline causes this LC circuit to resonate. But the antenna is lossy at its resonant frequiency, so the energy that is fed into it is radiated away.

If you drive it off resonance, some of that energy is not radiated, and is reflected back up the feedline, giving an SWR > 1.

It's probably best to start with standard transmission line theory first, to get a feel for why reflections occur at impedance mismatches in transmission lines:

http://en.wikipedia.org/wiki/Transmission_line

Have you had basic TL theory yet?

 

[david845]

Yes...the thing is I fully understand what all the texts say but they do not accurately answer my question. You can't just say rf works that way as if it is magic. Rf is still alternating current from accelerating charges. Tl theory doesn't exactly pick up where basic theory left off it simply says a transmission lines behaves as such without making the comparison of how it would behave if it were low freq. Ac

 

[david845]

To rephrase my question: if the open ends of a dipole behave like a capacitor that passes high frequency then why isn't the same true for current that reaches a break in the TL. Why does it reflect in the TL but radiate at the load when physically theyre the same thing 2 conductors separated by am insulator

 

[berkeman]

Yes...the thing is I fully understand what all the texts say but they do not accurately answer my question. You can't just say rf works that way as if it is magic. Rf is still alternating current from accelerating charges. Tl theory doesn't exactly pick up where basic theory left off it simply says a transmission lines behaves as such without making the comparison of how it would behave if it were low freq. Ac

A pair of conductors becomes a transmission line when the length of the line approaches a significant fraction of a wavelength of the excitation waveform. Let me try to give you a visual illustration. Consider a semi-rigid piece of rubber tubing, say 1/4" in diameter and 10 meters long. You are holding one end of it as you are floating weightless in the International Space Station. You can slowly raise and lower your end of the tubing, and if you do it slowly enough, the far end will follow your movements. But as you do it faster and faster, what you see is a traveling wave that propagates down the line, and since the far end is unterminated/free, you get a positive reflection that comes back to you and can create a partial standing wave.

And if you tie the other end to the wall of the ISS, when you move your end very slowly, all you do is make an angle down the tubing to the anchored end. But when you move it more quickly up and down, you send waves down the tubing and the negative reflections from the fixed end (short) also can help to set up partial standing waves.

That is why the frequency of the excitation matters in TLs. If the excitation is slow enough that the TL looks like its lumped parameters, then there are no TL effects like reflections.

To rephrase my question: if the open ends of a dipole behave like a capacitor that passes high frequency then why isn't the same true for current that reaches a break in the TL. Why does it reflect in the TL but radiate at the load when physically theyre the same thing 2 conductors separated by am insulator

An antenna is not an open circuit at resonance. It is an LC tank circuit with 100% loss. A break in the TL is a discontinuity in the Zo of the TL. An antenna is a matched load for the Zo of the TL.

Antennas are not magic. They are very real, physical things. What books are you trying to study antennas from? Stutzman & Thiele (sp?) is my main antenna theory book.

 

[david845]

Thanks for all the help I truly appreciate it!